The present invention relates to diagnosing aftertreatment hardware, and more particularly but not exclusively relates to diagnosing a fuel injector in an aftertreatment system. Emissions strategies for internal combustion engines often include aftertreatment components such as oxidation catalysts, NOx adsorbers, and/or particulate filters. Many aftertreatment components require periodic regeneration to function properly. Some aftertreatment systems include a fuel injector (or “closer”) in the exhaust flow to assist with regeneration or for other reasons. An undetected failure of the fuel injector may result in a failure of an aftertreatment component or a non-compliant aftertreatment system. Accordingly, there is a demand for further contributions in this area of technology.